1. Field of the Invention
The present invention relates to a duplex print apparatus designed to receive image data from a data processor such as a host computer or the like to print on both sides by means of a printer having a paper refeed stacker.
2. Description of Related Art
A laser printer is designed to form a latent image by picture elements on a photosensitive drum by way of a laser beam which is deflected by a polygon mirror rotating at high speed to obtain a visible image by toner developing and to fix it after transferring it onto a usual paper. Since fast modulation of the laser beam is possible, fast and high-quality (high-density) printing and graphic recording can be realized. Therefore, the laser printer is widely used as an output device of various data processing systems and image processing systems utilizing computers.
Meanwhile, in order to make good use of filing spaces and to save paper costs in conducting office work, a demand for duplex printing where the printing or copying can be made on both sides of the paper is becoming stronger. A most simple way to print on both sides of the paper by the laser printer is, for example, to print a first page on one side of the paper being fed, thereafter the paper is reversed in a paper circulation path in the laser printer so that a second page is printed on the reversed side of the same paper. In this method, however, since the circulation path becomes extremely longer as compared with the paper, it takes time to reverse the paper, thus requiring longer printing time.
For solving such a problem, it has been proposed to make the circulation path possible to pass a plurality of sheets, e.g. 3 sheets at a time, and to print first, third and fifth page successively on one side of each sheet, then to print second, fourth and sixth page successively on the other side of each reversed sheet. However, the number of papers present in the circulation path changes dependent on each printer, and even in the same printer, processing time of a character/picture processor for controlling the print portion changes dependent on contents to be processed, thereby the number of papers in the circulation path is also changed. Since printing conditions are changed dependent on each printer and its using condition as such, it is very complicated or practically impossible to control the sequence of pages of the image data transferred, on the host computer side.
Likewise, though it may be considered to transfer the image data in order from the host computer side and temporarily storing the data to be printed on the rear side of the paper on the printer side, in such a method, an image memory for several pages is required on the printer side, resulting in a high printer cost.